The present invention relates to a display device and a method of forming the same, and more particularly to an organic electroluminescent display device suitable for a flat display and a method of forming the same.
FIG. 1 is a fragmentary cross sectional elevation view illustrative of a conventional organic electroluminescent display device. The conventional organic electroluminescent display device is formed on a transparent supporting substrate 31. A transparent indium tin oxide film is provided which extends over the transparent supporting substrate 31. The transparent indium tin oxide film is patterned to be in the form of an anode 32 of the transparent indium tin oxide film. The patterning of the transparent indium tin oxide film may be carried out by photo-lithography and subsequent wet etching by use of a chemical including ferric chloride. An organic electroluminescent layer 35 is grown on the anode 32 by a vacuum evaporation method. A cathode is further formed by patterning over the organic electroluminescent layer 35.
If the patterning of the cathode is carried out by the photo-lithography and subsequent wet etching, then in removal of the photo-resist film and in the selective etching of the cathode, moisture is immersed into an interface of the cathode to the organic electroluminescent layer and further into the organic electroluminescent layer. As a result, the luminescent properties and life-time of the display device are remarkably deteriorated.
In order to avoid the above problems, it had been proposed to use a shadow mask for evaporations of organic electroluminescent materials and a cathode. In this shadow mask method, as shown in FIG. 1, a transparent electrode made of indium tin oxide is grown by a sputtering method on a transparent supporting substrate made of glass. The transparent electrode is patterned to a stripe shape to form the anode 32. A first organic electroluminescent layer 33 and a second organic electroluminescent layer 34 are sequentially laminated on an entire surface of the substrate. The organic electroluminescent layer 35 comprises laminations of the first and second organic electroluminescent layers 33 and 34. The shadow mask may be made of a metal and has stripe shielding portions 36 defined by slits. The shadow mask is aligned so that the slits cross over the patterns of the anode 32 so as to be in contact with the organic electroluminescent layer 35 over the transparent substrate 31. Thereafter, the cathode material is vacuum-evaporated on the organic electroluminescent layer 35 and on the stripe shielding portions 36 of the shadow mask.
Japanese laid-open patent publication No. 5-275172 discloses an organic electroluminescent display device and a method of forming the same. Anode lines of indium tin oxide are arranged at a constant distance to extend laterally over the transparent substrate. Walls are further provided at a constant distance to extend in a vertical direction to the anode lines. An organic electroluminescent layer is provided on the indium tin oxide anode lines and the walls over the transparent substrate. The walls have a height higher than the thickness of the organic electroluminescent layer. Thereafter, a metal source for vapor phase deposition is set at an angle for allowing each of the walls to be inserted between the source and the organic electroluminescent layer for deposition of the metal film to form a cathode.
The walls comprise a spin-coated negative photo-resist or a dry film. The spin-coated negative photo-resist or the dry film is exposed to a light pattern for selective cross-linking of the exposed part of the photo-resist to be insoluble and development and cleaning of the unexposed part of the photo-resist for selective removal thereof to form the photo-resist walls.
Alternatively, it is also disclosed that the photo-resist is selectively provided by patterning onto the areas surrounding the walls to be formed. Wall materials such as silica, silicon nitride, alumina are applied on the wall formation areas for subsequent solvent lift-off method to remove the photo-resist to form the walls.
If the metal for the cathode is vacuum-evaporated, the metals are removed along the walls inserted into between the organic electroluminescent layer and the walls whereby the cathode has the desired pattern.
The organic electroluminescent display device has pairs of anode and cathode in the form of pixels and a voltage of 5-20V is applied across the anode and cathode to apply a current through the organic electroluminescent layer for luminescence of the patterns.
The above conventional organic electroluminescent display device has a sandwich structure of the organic electroluminescent layer sandwiched between the anode and cathode. The anode and cathode are required to have the desired patterns for the luminescence of the organic electroluminescent layer by applying the voltage across the anode and cathode for the desired pattern display. In dot-matrix display, the electrodes are patterned so that the anodes and the cathodes are arranged to cross to each other in the form of matrix.
As described above, it is easy to carry out the patterning of the anode over the transparent supporting substrate. The problem is in the formation of the cathode 37 by patterning over the organic electroluminescence layer. If the cathode 37 is formed by the photo-lithography and subsequent wet etching, then in the removal of the photo-resist and in the selective etching to the cathode by the photo-resist, moisture is immersed into the organic electroluminescent layer and into the interface between the organic electroluminescence layer and the cathode whereby the luminescence properties and the life-time are remarkably deteriorated.
In order to avoid the above problem, there are a method of using a shadow mask over the organic electroluminescent layer for the formation of the cathode or a method of use of the photo-resist to form the walls which are to be used for patterning of the cathode.
In the former method of using the shadow mask, if the substrate size is large, then the dead weight of the shadow mask makes it difficult to have the contact with the organic electroluminescent layer at a center of the transparent substrate. As a result, the cathode is not isolated to form a short circuit. It is difficult to make an accurate patterning of the cathode. If the substrate size is not so large but the fine pattern is required, it is required to have the shadow mask thin. However, the thin shadow mask has an insufficient rigidity thereby resulting in difficulty in contact with the organic electroluminescent layer over the substrate.
In the above circumstances, it was proposed that the thin shadow mask is made of magnetic material so that the magnet is provided on an opposite surface to the surface of the transparent substrate on which the organic electroluminescent layer is provided whereby the stripe shielding portions 36 of the shadow mask are forced by the magnetic force to be in contact with the organic electroluminescent layer. The shadow mask contacts to the organic electroluminescent layer thereby damaging the organic electroluminescent layer. Since the organic electroluminescent layer is not more than 1 micrometers to suppress the driving voltage to not less than 20V, the damage 38 extends to the anode 32. If, as illustrated in FIG. 1, the damage 38 is positioned at a site where the cathode 37 should be patterned, then in the cathode formation process, the metal for the cathode is contact through the damage 38 to the anode 32 thereby forming a short circuit between the anode and cathode. As a result, the necessary voltage for luminescence is not applied to the organic electroluminescent layer. Thus, no luminescence appears on the display pixel. This reduces the yield of the display device.
On the other hand, in the later method of using the walls of the photo-resist, it is required that the walls are positioned between the organic electroluminescent layer and the evaporation metal source for the cathode formation, for which reason a large vacuum growth chamber is required for patterning of the cathode over the large transparent supporting substrate. This method is somewhat expensive.
Adjacent two cathodes are distanced from each other by an isolation distance of the sum of the width of the wall and the width of the region of the organic electroluminescent layer on which the wall sandwiched between the evaporation metal source for the cathode formation and the organic electroluminescent layer is projected. For this reason, in the large scale display substrate, the isolation distance is different between the close and far positions to the evaporation metal source for the cathode formation.
Further, in the substantially parallel pattern to a straight line connecting the substrate and the evaporation metal source for the cathode formation, no wall projecting portion exist on the organic electroluminescent layer whereby it is impossible to carry out the patterning of the cathode.
In the above circumstances, it had been required to develop a novel organic electroluminescent display device free from the above problems.